Lewis Hamilton obviously did this song because he enjoyed doing it, so there's nothing wrong with doing something that you enjoy. Him being on twitter or whatever else you think is distracting him from racing is plainly incorrect. After Lewis, or any other driver for that matter puts their visor down before a race, they obviously are totally focused on driving the car and doing the best job possible.
As for supposedly improving his car.. Is he the Technical director of Mercedes? No. Is he the Head of production department? Head of engine and electronics? Chief designer? Head of aerodynamics? No no no and no.
But yes, in regard to developing the car, he can give ideas and give feedback. But I find it kind of baffling that you say that he would rather spend his time releasing a single than supposedly help "develop" his car for 2014.